1. Field of the Invention
The present invention relates to a charge pump circuit which utilizes a charging voltage of a charged capacitor to shift a power supply voltage, thereby producing an output voltage.
2. Conventional Art
Charge pump circuits are used in many types of conventional circuits. Particularly in various types of portable devices, power supply voltage is limited because a battery power supply is used. On the other hand, there is a conflicting demand that a higher power supply voltage be made available in order to facilitate operations of desired circuits. Thus, a charge pump circuit is often used to boost the voltage of a battery power supply. Such a charge pump circuit utilizing a capacitor is disclosed in, e.g., Japanese Patent Laid-open Publication No. Hei 7-298607. Further, a negative power supply may be required for circuit operations in some cases, a charge pump circuit is used in such cases.
Here, in such a charge pump circuit, during a changeover operation of a switch which controls charging and shifting of a capacitor, a large current change occurs in a short time, at the time of changeover of the switch in particular. Because this change is applied to a power supply or GND (ground), noise is generated.
In particular, when such noise is superposed on a video signal which is used to display an image on a display, this noise disadvantageously affects the appearance of the displayed image. When a signal which is asynchronous with the video signal is used as the above-described switching signal, although it may appear that the signal has not been damaged, because a switching clock has a fixed cycle and noise is present at a position where an image is displayed, there is a problem that an an oblique stripe pattern may appear on a monitor, depending on when the image is viewed. When a horizontal synchronizing signal or the like is used for a switching signal of the switch in the charge pump circuit at a position where a picture is displayed, noise is superposed on a specific position and a vertical line is generated in the display displayed on the display unit.